The present invention relates to vacuum valves, and more particularly to valves for use in a continuous vacuum drying apparatus, such as a continuous freeze dryer.
In this equipment, valves with small bores are inadequate to deal with the large volumes of gases involved. Vacuum valves must present low impedance to the vacuum pump for maximum air removal capacity, and also low impedance for water vapor flow coming from the drying chamber. Therefore, valves with a minimum internal diameter of one half inch are needed for efficient fluid flow, and then on upwards to three inches I.D. and larger. The valves must operate on a continuous basis, and do it reliably over a period of years.
The problem that is addressed here is best exemplified by continuous vacuum applications, as for example, an apparatus for continuous freeze drying. In a standard, non-continuous freeze dry apparatus aqueous samples to be dried are first frozen and then are subjected to a high vacuum and controlled heat input. Under these conditions the water content of the sample sublimes off, ultimately leaving a dry, porous product which is substantially unaltered by the drying process. The freeze dry method is used extensively for the high quality preservation of pharmaceuticals such as vaccines, antibiotics, vitamin preparations, etc.
The freeze dry apparatus basically consists of three components. First, a vacuum chamber to contaim the product to be dried; second, a water vapor trap to sequester the sublimating water vapor; and third, a vacuum pump to provide the required high vacuum for sublimation to efficiently occur.
For the process to be continuous at least a fourth component, i.e. a second water vapor trap, must be added. During the drying process, if means are provided for continually adding product to the drying chamber, the first trap will eventually exhaust its water holding capacity. At this point it must be temporarily be disconnected from the system, and the second trap connected in its place while the first trap is being defrosted or otherwise regenerated for subsequent reconnection to the system at the point that the second trap is exhausting its water holding capacity.
Except for small scale research equipment the fluid conduits used in freeze drying have necessarily large internal diameters of the order of 1 inch, 2 inches, 3 inches or even larger. This is to provide low impedance removal of air from the sytem by means of the vacuum pump, and efficient flow of water vapor from the product chamber to the vapor trap. Therefore, vacuum valves that are needed for a continuous process must have large bores and offer low flow resistance. A variety of such valves are commercially available, ofter referred to as "gate", "flap", or "disk" valves. Solenoid operated disk valves are often a preferred valve for high vacuum systems. They can be maintained in a fully open or fully closed position until energized, depending on the application.
Although valves of the above mentioned types can be employed successfully in a continuous vacuum system, they are expensive to manufacture. Since a number of these valves would be required to permit continuous operation they add significantly to the cost of the complete equipment.
Servicing these valves is difficult. Gross defects within the valve cannot be seen from the outside of the valve since they are normally fabricated in opaque materials, such as metal. Subtle defects within the valve also require removal of the valve in order to take it completely apart to carefully examine all of the components.
Another disadvantage of these valves, when they must have large internal diameters, is their large bulk. Connecting a number of them within the confines of a cabinet often presents difficult problems of spatial arrangement of the components of the cabinet.
Accordingly, it is an object of the invention to provide an inexpensive vacuum valve.
Another object is to provide an inexpensive vacuum valve for continuous vacuum procedures.
Another object is to provide a vacuum valve that is easy to service.
Another object is to provide a vacuum valve that is extremely relaible in operation.
A further object is to provide a vacuum valve that is compact and easy to install in confined spaces.